The Spanish realm C'thun (about the only one censused) seems evenly mixed with Spanish and English guild names.

And what about servers with large populations of people who's native language isn't one of Blizzards supported one? Are they using supported language words for guild names.. or slipping in native language names?

A) The language pack in use does not really define guild names in any way, shape or form. Nor does it bind one to a specific language realms... I run my WoW in English, yet have and play significant amount of characters on German realms.

B) French servers are dominated by French players, the Spanish ones by the Spanish... But German servers see large numbers of Austrians and Swiss... and due to Germany's population structure, also a significant amounts of Turks on several of them.

And the EU-ENG ones... well, they are a mishmash... Hakkar and Crushridge have been invaded by the Italians, Stormscale by Swedes, etc. And they, like no other group, extend outside EU area... I have met Canadians, Americans, South-Africans, Qutarians, Saudi-Arabians, etc. Some Russians also prefer the (in their opinion and most likely fact) superior EU accounts to their own, etc.

English is the most common shared language... BUT... the truth is that in the EU native English speakers are a minority. And if you see someone speaking really good English... Well, the odds are that you are seeing an (older) Dutch, Norwegian or even a Finnish player.

C) There are plenty of guild names that don't correspond to any current nation or real language. Latin ones being one, languages invented by Blizzard another and then there are the abbreviations, from which one can't tell the language... For example OMG can stand for Oh mein Gott, One man guild and several other things that currently escape me.

And then... there are ones that are simply a mishmash, jokes or gibberish in themselves...

Thanks for the information!
The reason for my question was to find realms with people who might use their native language when selecting character names. So from what you have said I know that I should concentrate on the EU-Eng realms.